PROJECT SUMMARY/ABSTRACT Males and females have striking differences in development, health and disease, but the underlying mechanisms remain poorly understood. At the genetic level, different sex chromosome constitutions, XX in females and XY in males, result in differential expression of sex chromosome genes. Autosomal genes are also differentially expressed between the sexes, but, since autosomal DNA content does not vary by sex, this difference is evidently due to epigenetics. Interestingly, sex chromosome genes encode transcription factors and chromatin modifiers, which may have different expression levels and/or activities in males and females. Thus, I hypothesize that the actions of epigenetic and transcriptional regulators encoded by the X and Y chromosomes create sexually dimorphic expression across the genome. In Aim 1, I will ascertain whether epigenetic and transcriptional programs depend on sex chromosome dosage using cells from males and females varying in their sex chromosome complement (XO, XX, XY, XXY, XYY, and XXX). In Aim 2, I will assess activities of candidate regulatory genes on the sex chromosomes in males and females, and individually manipulate them to investigate their functional roles in epigenetic and transcriptional sex differences. This work will increase our mechanistic understanding of differences between the sexes, and motivate future research on sex-biased diseases.